(Props to @JLG for making me look this up.) Signatory need not refer simply to a party in a contract. While some dictionaries define it that way:
NOAD: a party that has signed an agreement, esp. a country that has signed a treaty
Collins English Dictionary (via Dictionary.com): person who has signed a document such as a treaty or contract or an organization, state, etc, on whose behalf such a document has been signed
Cambridge Dictionaries Online: a person, organization or country which has signed an agreement
Oxford Dictionaries Online: a party that has signed an agreement, especially a state that has signed a treaty: Britain is a signatory to the convention
Others define it more broadly.
M-W: a signer with another or others signatories to a petition; especially : a government bound with others by a signed convention
Random House Dictionary (via Dictionary.com): a signer, or one of the signers, of a document: France and Holland were among the signatories of the treaty.
Wikipedia: The writer of a signature is a signatory.
Notice, however, that these last three seem to be more layman's dictionaries than comprehensive ones. In light of this, it would seem that it is all right to use signatory in informal or non-official usage, but that a better word might be needed for official use.
Disclaimer: I am not a lawyer.